The oxygen content of the exhaust gas is determined by the oxygen measuring probe and the value determined in this manner is supplied to a control arrangement which controls to a pregiven air/fuel ratio. The oxygen measuring probe is operationally ready only above a minimum operating temperature. In this way, the control of the air/fuel mixture via the oxygen measuring probe is only then possible when the probe has reached its operating temperature. Only then can there be a control to an optimal air/fuel mixture, for example, with respect to a low emission of toxic materials. In order to maintain low emission values, the operating temperature of the oxygen measuring probe should reach its operating temperature as quickly as possible after the internal combustion engine is started. The probe is heated by the exhaust gases and this heating is accelerated by an electric probe heater for the above-mentioned reasons. The electric probe heater is then also required when, for example, the heat capacity of the exhaust gas is inadequate such as during idle in order to maintain the probe at the operating temperature or for an overrun operation of long duration.
It is necessary to monitor the operability of the probe heater for obtaining a low emission of toxic materials. Numerous methods are known to detect one or more fault conditions, namely: circuit interruptions, short circuits and shunts. The check of the operational readiness of the probe heater is made, for example, in the following ways: from the current flow through the probe heater detected by means of a measuring resistor (U.S. Pat. No. 5,285,762); via the output signals of the probe (U.S. Pat. Nos. 4,170,967 and 5,054,452); via the warm-up performance of the probe (U.S. Pat. No. 5,090,387); or via the probe temperature which can be determined in various ways such as from the internal resistance of the probe (U.S. Pat. No. 4,419,190); or, with a temperature sensor (U.S. Pat. No. 3,915,828).